Solid-state memory in the form of flash memory has recently become the storage of choice in a variety of mobile & handheld devices, notably information equipment and consumer electronics products. Unlike RAM (random access memory) which is also solid-state memory, flash memory is non-volatile which retains its stored data even after power is turned off. In spite of the higher cost, flash memory is increasingly being used in mass storage applications where information is stored semi-permanently in file format. Conventional mass storage, based on rotating magnetic media such as hard drives and floppy disks, is unsuitable for the mobile and handheld environment. This is because disk drives tend to be bulky, are prone to mechanical failure and have high power requirements. These undesirable attributes make disk-based storage impractical in most mobile and portable applications. On the other hand, flash memory is ideally suited in the mobile and handheld environment because of its small size, low power consumption, high speed and high reliability features.
While some of the early products have been using built-in flash memory chips, current products using flash memory are mostly in the form of a removable memory card. The removability feature has the added advantage of memory-capacity flexibility, reduced cost for the product itself, and portability and ease of information transfer from one host system to another.
One common usage of removable memory card is therefore in mobile computing such as laptop and palmtop computers, PDAs (personal digital assistants). In addition to providing application programs and data storage on these mobile hosts, the removable memory card provides a convenient way to exchange data and files between different hosts. For example, by moving the memory card from a mobile host to a desktop host, data in the field can be easily synchronized with that on the desktop. In the case of a limited-featured mobile host, the transferred data can be better manipulated on a full-featured desktop computer.
A common interface has been established for memory cards. It is known as the PC card standard or PCMCIA interface as specified by the Personal Computer Memory Card Interface Association. The standard establishes both the mechanical and electrical characteristics of the memory card interface. It is designed specifically for solid-state memory application although the interface has, since its inception a few years ago, been adapted to become a general purpose interface for card-based devices such as modems, network adapters and even hard drives. It has a size about that of a credit card and comes in Type I, II and III with increasing thickness from a couple of millimeters to several millimeters. There is a 68-pin socket on one card edge for mating with a connector located on a PC card slot of a host device. While PC card slots have become standard equipment on laptop and notebook computers, they have hardly been deployed on desktop personal computers (PCS). Thus, for a desktop computer to communicate directly with a PC card, a PC card slot must first be installed.
Currently, there are two main flash memory architectures that conform to the PC card standard, namely ATA flash and Linear flash.
ATA flash, with its 512 bytes flash sector architecture, mimics the DOS file structure of an ATA (Advanced Technology Attachment, i.e., one that communicates with a standard PC ISA bus) hard drive. In addition, the memory operation is under the control of a dedicated intelligent memory controller which also handles defect management and memory optimization. To a PC running DOS and MS Windows(R) operating system, a card with an ATA controller appears like another disk drive. This means that, under the standard Card and Socket Service drivers included with Windows 95(R), no additional software is needed to use ATA flash.
Linear flash on the other hand does not require to work with a dedicated memory controller. Typically, it relies on the intelligence and processing power of the host computer to handle the memory operations. It stores and communicates data in large blocks each having a size much larger than the DOS disk sector of 512 bytes. Its address space can be compatible with that of the system memory and could be seen as an extension of system RAM or ROM. Thus, it is possible to execute application programs directly from it rather than having to first load the application program into the system memory. When used as mass storage, Linear flash requires a software layer to translate data in blocks form to the file format of the operating system and vice versa. This software layer is called Flash File System (FFS). An alternative software layer is called Flash Translation Layer (FTL).
Thus, when plugged into a host PC system, some PC memory cards are immediately recognized as another disk drive while others require addition software and/or hardware to operate.
Removable memory card is also being incorporated into a new class of "smart" consumer electronics products. With products like smart cellular phones and pagers, the memory card is used to store data such as address book and personal information, or to record voice, e-mail, faxes, and pager messages, or to cache stock quotes and news feed. Again, the removable memory cards allow data to be portable between similar host devices and for the field data to be exchanged with that of the desktop computers.
With smaller handheld devices, the need arises for even smaller removable memory cards. A number of smaller form factor card standards have emerged. Three competing standards, namely CompactFlash, Miniature Card and SSFDC are being supported respectively by the CompactFlash Association (CFA), the Solid State Floppy Disk Card Group (SSFDC) and the Miniature Card Implementers Forum (MCIF). More recently, yet another card standard known as MultiMedia card (MMC) is being announced. All these cards are about postage stamp size with different electrical interfaces and slight difference in dimensions. Unlike one main PC card standard, there are then several smaller form factor card standards each specifying somewhat difference mechanical and electrical interfaces. A device or product will need a particular type of card slot for a particular type of smaller form factor card. However, most of the smaller form factor cards are provided with an adapter to conform to the PC card standard. Typically, the adapter is in the form of a Type II PC card for embedding the smaller form factor card into. In this way, they may be used in an existing PC card (PCMCIA) slot, possibly with addition software and hardware as required.
Removable memory card is rapidly entering the consumer marketplace because of widespread adoption by consumer electronics products such as digital cameras and digital audio recorders. The memory card replaces conventional photographic film or magnetic tape and provides virtually an unlimited amount of renewable recording media. For example, the memory card records image frames captured by a digital camera in the form of a compressed image file. A 4 MB flash memory card may hold approximately 40 frames (VGA resolution 640 by 480 pixels) at the rate of 100 KB per compressed file. When a card is filled up, a new or erased one could quickly replace it. The added advantage over conventional film is that the memory card could be erased and reused again and again when the existing data on it is no longer needed or has been archived to another storage.
Once the image files have been stored on the memory card, they can typically be called up frame by frame and displayed on a built-in small LCD screen provided on the camera. When hard copies of the recorded images are to be made, the camera sends its output to a specialized photo printer dedicated for this purpose. More common solutions have been to move the memory card into a desktop or a laptop host computer equipped with a slot for receiving such memory card. The image files can then be displayed on the computer with or without further manipulation before being printed on an attached standard printer.
Currently, a smaller form factor memory card may have a capacity as high as 40 MB and this capacity will be likely doubled with every new generation of flash memory devices. With this amount of capacity, it is a viable storage for multimedia information. Consumer products such as digital voice recorders and digital audio recorders will be able to record voice and audio information onto a removable memory card. Similar to the digital camera case, the recorded audio information on a memory card can be played back on its original host or on a desktop or laptop host.
Thus in most cases, the memory card is intended to be used in conjunction with its original. host device or with a PC host that is equipped with a compatible card slot for receiving the card. However, because many of the applications have wide consumer appeal, it will be desirable to process, render and playback the information recorded on a memory card in a consumer environment instead of the original host or a PC host. Also, it will be desirable to record audio, video, and images from consumer electronics machines or from settop boxes for Internet access or from "thin" "network computers" (NC).